It is known to automatically control the windshield wipers on a vehicle by using a moisture sensor coupled to a wiper system to actuate the system when rain or other water is detected. The sensor is preferably on the windshield surface and wiped during wiper operation. Usually, such sensors are resistive or capacitive and comprise apattern of spaced apart electrodes on the outside surface of the windshield which provides a normally high or open circuit impedance to a sensor circuit electrically connected to the electrodes. In the presence of moisture from rain or the like, the spaced apart electrodes are bridged by films or drops of water which decreases the impedance across the sensor. This acts as a signal to the sensor circuit which turns on the wiper motors that activate the wiper blades so as to remove moisture from the windshield.
Two classes of materials have heretofore been used to provide the sensor electrodes. One comprises compositions of metal particles and a glass frit which is fused to the windshield. A shortcoming of these compositions is that they lack resistance to abrasion, hence tend to be quickly removed under wiper action.
The other class of materials is that of tin or other metal oxides which are normally sprayed as compounds onto the windshield through an appropriate mask. A shortcoming of these materials is that after they are applied to the windshields, additional processing steps are required to achieve the desired adherence of the material to the windshield and the desired conductive characteristics of the electrodes they form. These additional processing steps are undesirable in that they increase manufacturing costs.
Accordingly, a need exists for improved sensor electrodes which are highly resistive to abrasion and which can be economically applied to windshields.